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Short answer
On a Gram stain, Streptococcus pneumoniae (the pneumococcus) is gram-positive: it stains purple and typically appears as lancet-shaped cocci in pairs. The CDC says that appearance is “suggestive of pneumococcal infection”, not proof. Definitive diagnosis generally relies on isolating the organism from a normally sterile body site.
Definition and scope
A Gram stain sorts bacteria by cell-envelope structure. Gram-positive cells keep the crystal violet-iodine complex and look purple; gram-negative cells lose it at the decolorizing step and take up the red-pink safranin counterstain instead. The method itself, with reagents and timings, is covered in the CASRAI guide to Gram stain procedure, reagents and results. This page covers only what the pneumococcus looks like on a smear and how far that finding can be pushed.
The CDC Pink Book describes pneumococci as “lancet-shaped, gram-positive, facultative anaerobic organisms” that are “typically observed in pairs (diplococci)” but may also occur singly or in short chains, and notes that most are encapsulated, with surfaces made of complex polysaccharides. The CDC’s clinical overview (page dated 25 February 2026) adds that the species has more than 100 known serotypes. The stain does not distinguish serotypes.
The same organism appears under several search phrasings: pneumococcal gram stain, strep pneumoniae gram stain, S. pneumoniae gram stain, and pneumococcal pneumonia gram stain. They all refer to this one organism and one question, what the stain shows.
What S. pneumoniae looks like on a Gram stain
Colour: gram-positive, purple
The organism retains the primary stain, so cells read purple or violet. Gram-negative organisms, by contrast, stain pink to red. The CASRAI procedure guide describes over-decolorizing as a way a gram-positive organism can be made to read as gram-negative.
Shape and arrangement: lancet-shaped diplococci
The CDC describes pairs of lancet-shaped cells. The CDC/WHO meningitis laboratory manual (WHO/CDS/CSR/EDC/99.7) likewise lists S. pneumoniae as gram-positive, lanceolate diplococci, sometimes in short chains. For the side-by-side CSF comparison with Neisseria meningitidis and Haemophilus influenzae, see the H. influenzae Gram stain guide and the N. meningitidis Gram stain guide; both gram-negative organisms are separated from the pneumococcus by Gram reaction as well as shape.
Gram stain vs. culture, antigen and molecular tests
The comparison below is the practical point of the page: each finding supports a different strength of statement. The source for each row is named in the last column.
| Finding | What the CDC says it supports | Source |
|---|---|---|
| Lancet-shaped gram-positive diplococci on a Gram stain | “Suggestive of pneumococcal infection” | Pink Book, Ch. 17 |
| Sputum smear with more than 25 white blood cells and fewer than 10 epithelial cells per low-power field, and a predominance of gram-positive diplococci | The criteria for a sputum Gram stain consistent with pneumococcal pneumonia; interpretation is difficult because normal nasopharyngeal bacteria can contaminate sputum | Pink Book, Ch. 17 |
| Organism isolated from blood or another normally sterile body site | The basis of a definitive diagnosis | Pink Book, Ch. 17; Clinical Overview |
| Urinary antigen test (C-polysaccharide, immunochromatographic membrane) | A non-culture test, described for adults with community-acquired pneumonia | Pink Book, Ch. 17 |
| Culture-independent molecular methods | Available as an additional route to identification | Clinical Overview |
The Pink Book names blood, cerebrospinal fluid, middle ear fluid, joint fluid and peritoneal fluid as the kinds of sites involved. A smear of sputum is not a sterile-site specimen, which is why the sputum criteria exist at all.
Where it matters for labs and clinics
Three practical points for labs and clinics.
- Report the smear as a morphology, not an identification. The wording that the CDC itself uses is “suggestive”. A report that says “gram-positive diplococci” carries a different weight from one that says “S. pneumoniae“.
- Check specimen quality before reading too much into a sputum smear. The 25 and 10 per-field figures are the CDC’s stated quality markers, and the Pink Book warns that normal nasopharyngeal bacteria make sputum interpretation difficult.
- Plan for follow-up testing. The CDC places definitive diagnosis on isolation, and urinary antigen and molecular methods are further options. On blood agar, S. pneumoniae produces the alpha-hemolytic green discoloration described in the CASRAI guide to alpha hemolysis on blood agar.
For the optics side of reading a smear under oil, see microscope objective selection.
Common misunderstandings
- “Staphylococcus pneumoniae.” This is a misspelling. The organism is Streptococcus pneumoniae. Staphylococci typically cluster in grape-like groups on a smear, while the pneumococcus is described as pairs; both are gram-positive, so colour does not tell them apart, and arrangement is the clue (see the CASRAI procedure guide).
- “A positive Gram stain means pneumococcal pneumonia.” The CDC calls the appearance suggestive and places definitive diagnosis on isolation from a normally sterile site.
- “Any sputum smear with diplococci is diagnostic.” The Pink Book notes that interpretation of sputum is difficult because of normal nasopharyngeal bacteria, which is why the quality criteria exist.
- “A stain tells you the serotype.” It does not. The species has more than 100 known serotypes.
FAQ
What is a Gram stain?
A staining method that separates bacteria into gram-positive (purple) and gram-negative (pink to red) groups based on cell-envelope structure. It is a first look at morphology, not an identification.
What is a Gram stain test?
It is the same method applied to a clinical or laboratory specimen, read under oil immersion for colour, shape and arrangement. A result is usually reported as a morphology such as “gram-positive diplococci”.
What is the purpose of a Gram stain?
To give an early, low-cost indication of what kind of bacteria are present. For the pneumococcus the CDC treats it as suggestive evidence that must be followed by isolation or other testing.
How does a Gram stain work?
Gram-positive bacteria have a thick peptidoglycan wall that holds the crystal violet-iodine complex. Gram-negative bacteria have a thin wall plus an outer membrane, lose the complex when decolorized, and take up safranin. The full explanation is in the procedure guide.
How to Gram stain bacteria?
The method uses crystal violet, iodine, a decolorizer and a safranin counterstain, applied to a fixed smear. Timings and technique are in the CASRAI procedure guide and in your laboratory’s own SOP; this page does not give a step list.
What color does gram negative stain?
Pink to red, from the safranin counterstain. S. pneumoniae is gram-positive and stains purple.
Is Streptococcus pneumoniae gram-positive or gram-negative?
Gram-positive, per the CDC.
Sources
- CDC, Epidemiology and Prevention of Vaccine-Preventable Diseases (Pink Book), Chapter 17: Pneumococcal Disease, reviewed 1 May 2024. cdc.gov/pinkbook. Date checked: 29 October 2026.
- CDC, Clinical Overview of Pneumococcal Disease, page dated 25 February 2026. cdc.gov/pneumococcal. Date checked: 29 October 2026.
- WHO/CDC, Laboratory methods for the diagnosis of meningitis caused by Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae (WHO/CDS/CSR/EDC/99.7, 1999). who.int. Date checked: 29 October 2026.
